Field of the Invention
The present invention relates to a photovoltaic device comprising three organic layers disposed between two electrodes, of which at least one of said electrodes is light transmittable, wherein the three organic layers consist of and are arranged either as a layer of organic electron acceptor (hereinafter referred to as "OEA") material (a), a layer of organic electron donor (hereinafter referred to as "OED") material (b) and a layer of OED material(c) different from OED material(b); or as a layer of OED material(d), a layer of OEA material (e), and a layer of OEA material(f) different from OEA material (e); arranged in this order from the light-incident side.
Various photovoltaic devices using organic materials as functional materials have hitherto been studied in an attempt to provide an inexpensive and less toxic photovoltaic device which is difficult to obtain by using single crystal, multicrystal or amorphous silicon.
Since the purpose of a photovoltaic device is to convert light energy into electric energy (voltage.times.current), a primary measure of the device is its conversion efficiency.
To generate a photocurrent, an internal electric field is necessary and several device structures to generate such a field are known. Data on the conversion efficiency of some exemplary device structures composed of organic functional materials are illustrated below.
(1) Schottky junction or MIS junction
In these junctions an internal electric field generated between a metal and a semiconductor is utilized. As organic semiconductor materials, merocyanine dyes, phthalocyanine dyes, etc. have been used.
When white light with an intensity of 78 mW/cm.sup.2 was irradiated onto an Al/merocyanine/Ag device, a conversion efficiency of 0.7% (Voc=1.2 V, Jsc=1.8 mA/cm.sup.2, ff=0.25) was reported, see A. K. Ghosh, et al. J. Appl. Phys, 49, 5982 (1978).
However, organic semiconductors with a high conversion efficiency used in devices of this type are limited to p-type materials. Accordingly, materials of low work function such as Al, In and Mg must be used for the electrode, but, unfortunately, such materials are readily oxidized.
(2) Hetero on junction utilizing n-type inorganic semiconductor/p-type organic semiconductor junction
An internal electric field generated upon bonding an n-type inorganic semiconductor and a p-type organic semiconductor is utilized in this junction. CdS, ZnO or the like are used as the n-type material and merocyanine dyes, phthalocyanine dyes, etc. as the p-type organic semiconductor material.
When AM-2 light of 75 mW/cm.sup.2 is irradiated onto an indium tin oxide (hereinafter referred to as "ITO")/electrochemically deposited CdS/ chloroalminiumphthalocyanine chloride/Au device, the conversion efficiency is 0.22% (Voc=0.69 V, Jsc=0.89 mA/cm.sup.2, ff=0.29), see A. Hor et. al., Appl. Phys. Lett., 42, 15 (1983).
(3) A device utilizing organic/organic material hetero junction
An internal electric field generated upon the bonding of an OEA material and an OED material is utilized in this device.
As examples of the OEA material, dyes such as malachite green, methyl violet and pyrylium, and condensed polycyclic aromatic compounds, such as, flavanthrone and perylene pigment are used and as examples of the OED material, phthalocyanine pigments, merocyanine dyes, etc. are used.
When AM-2 light of 75 mW/cm.sup.2 is irradiated onto an ITO/copper phthalocyanine/ perylene pigment/Ag device, 0.95% of conversion efficiency (Voc=0.45 V, Jsc=2.3 mA/cm.sup.2, ff=0.65) has been reported, see C. Tang, Appl. Phys. Lett., 48, 183 (1986). This value is the highest of the photovoltaic devices employing organic materials so far. Further, in Japanese Patent Publication No. 62-4871 (1987) of the same inventor, 1% of conversion efficiency (Voc=0.44 V, Jsc=3.0 mA/cm.sup.2, ff 0.6) has been reported for the device with the same structure but containing a different kind of perylene pigment.
The conversion efficiency of a photovoltaic device using an organic material is lower than that using an inorganic semiconductor. One of the most important reasons for this phenomenon is that the short circuit-photocurrent (herein referred to as "Jsc") is low. When irradiated with white light of 75 mW/cm.sup.2, at least 10 mA/cm.sup.2 Jsc is necessary. The Jsc value of the device described above is much lower than this. This is ascribed to a low quantum efficiency and a narrow photosensitive wavelength region. The photosensitive wavelength region should preferably extend from 400 nm to longer wavelengths to provide a region as wide as possible; however, photosensitive wavelength regions of commercial devices are often at shorter wavelengths and narrower regions than the desired values.
Furthermore, the fill factor (herein referred to as "ff") is frequently low. Low ff can be attributed to a decrease in the quantum efficiencies exhibited by an organic semiconductor at low electric field. Therefore, to improve the ff value, it is preferable to develop a device consisting of a structure which can form an intense internal electric field and does not suffer a decrease in efficiency. Further, ff will be increased if a device structure is constructed in which formed carriers can easily reach an electrode without an energy barrier. That will lead to an improvement in the open circuit voltage (herein referred to as "Voc"), but so far these factors have not been sufficiently considered.
Additionally, most conventional photovoltaic devices employing organic layers are beset with several problems, such as, the chemical instability of the electrode material.
Bearing in mind the points described above, known device structures (1), (2) and (3) described before will again be considered.
(1) Schottky junction or MIS junction
Although a Voc as high as 1 volt can be obtained, the light transmission of the electrode is low, because of the necessary metal electrode. Actual light transmittance is 30% at best and is usually about 10%. In addition, the materials are poor in oxidation resistance. Accordingly, high conversion efficiency and stable characteristics are not obtained with this device structure.
(2) Inorganic semiconductor/organic semiconductor hetero pn junction
Since photocarriers are mainly formed in an organic material layer, such a junction cannot avoid a limitation due to its photosensitivity. This limitation results because an organic material layer is usually formed with a single material and because an organic semiconductor having an intense absorption, from 400 to, for instance, 800 nm wavelength does not exist at present. Accordingly, although the device of this structure can overcome the problems of light transmittance of an electrode on a light-incident side and the stability of the electrode, high conversion efficiency cannot be achieved because of its narrow photosensitive wavelength region.
(3) Organic material/organic material hetero pn junction
This junction is the most desirable one at present relative to the two other structures described above. Light can be irradiated through a transparent electrode and, as photocarriers can be formed in an interfacial region existing in the two different layers, the photosensitivity region can also be widened. Actually, it can be assumed from Tang's report described above that carriers are formed by the perylene pigments at 450-550 nm wavelength and by copper phthalocyanine at 550-700 nm wavelength. Further, since the ff is greater than with other device structures, one can assume that a high internal electric field is formed. However, Tang's device does have some disadvantages.
One disadvantage is the occurence of pinholes due to the thinness of the organic material layer (it is described in the patent that a thickness of 300 to 500 .ANG. is desirable). Also in our experiment, short circuits between two electrodes possibly attributable to pinholes are observed with relatively high frequencies. Also, the electrode area is described as 0.1 cm.sup.2 in Tang's report and a significant problem will arise regarding the low yield of the device in the case of practical use which requires an electrode having an area larger than 1 cm.sup.2.
A second disadvantage concerns the electrode material. In Tang's invention, the electrode must be in ohmic contact with each of the organic layers. Also in his report, it is described that the Voc is reduced in a device structure, in which the sequence order of organic layers is reversed. This is estimated to be attributable to a deterioration of the ohmic contact described in his invention. On the other hand, in a device structure in which the ohmic contact is sufficient, there is another problem with the stability of the metal material of the electrode, because a metal having an ohmic contact with an OEA material will necessarily have a low work function. Actually, In, Ag, Sn and Al are exemplified in the patent literature, all of which are readily oxidized.
The present inventors have undertaken a program of research directed to obtaining a device structure which can overcome the disadvantages of a photovoltaic device having organic layers and which has a relatively high conversion efficiency as an organic photovoltaic device. The inventors have discovered that a device having the following structure can meet these aims: a photovoltaic device containing a portion comprising three organic layers disposed between two electrodes, of which at least one of said electrodes is light transmittable, wherein the three layers consist of and are arranged as a layer of OEA material (a), a layer of OED material (b) and a layer of OED material (c) different from OED material (b); or as the same three layers except that an OEA material is to be read as an OED material and an OED material as an OEA material, respectively; arranged in this order from the light-incident side.
Based on these findings, they have completed the present invention.